Make the cream sauce first by sautéing the celery in 1 tablespoon of butter until soft, about 10 minutes. In a pot, whisk the cornstarch into the milk, then add the remaining ingredients except the celery. Bring to a boil and simmer for one minute. Add sautéed celery.
Grease a 3 quart casserole dish. In a bowl, combine all the the salmon casserole ingredients thoroughly, then mix in the celery cream sauce. Pour into greased casserole dish. Bake at 350 degrees for 45 minutes.
Notes
Modern twist: if you're looking for a shortcut, you can substitute a 10-½ oz. can of condensed cream of celery soup in place of the celery cream sauce. If you do that, reduce the milk in the salmon casserole from ¾ to ½ cup. If you don't have 2 tablespoons of fresh parsley, you can substitute 1 tablespoon of dried parsley instead.